For some years, different types of cellular networks for wireless communication have been developed to provide radio access for various wireless terminals in different areas. The cellular networks are constantly improved to provide better coverage and capacity to meet the demands from subscribers using services and increasingly advanced terminals, e.g. smartphones and tablets, which may require considerable amounts of bandwidth and resources for data transport in the networks. A limiting factor for capacity of cellular networks is the amount of available radio resources, e.g. defined by a combination of time, frequency and/or code depending on the access technology used, and the capacity of a network may be improved by more efficient usage of such radio resources.
In this disclosure, the term “User Equipment, UE” is used to represent any user-controlled wireless terminal or device capable of radio communication including receiving downlink signals transmitted from a radio node of a cellular network. Without limitation, the UE may be a non-stationary device operated by a user or an automatically operating device sometimes referred to as a Machine-to-Machine (M2M) device. Further, the term “network node”, represents any node of a cellular network that can communicate uplink and downlink radio signals with UEs. The network node in this description may also be referred to as a base station, NodeB, e-nodeB, eNB, base transceiver station, etc.
In the Third Generation Partnership Project, 3GPP, two different states of Radio Resource Control, RRC, have been defined for UEs communicating data, referred to as the “CELL_FACH” state where FACH=Forward Access Channel, and the “CELL_DCH” state where DCH=Dedicated Channel. Of these two states the CELL_FACH state involves communication on the FACH which is a common channel with radio resources that can be shared by multiple UEs, while the CELL_DCH state involves communication on dedicated channels with radio resources being exclusively reserved and occupied by each UE until its session ends.
In the CELL_FACH state, each UE needs to perform a random access procedure to obtain some of the shared radio resources for uplink communication. The radio resources used for UEs in the CELL_DCH state may easily become congested in a cell since relatively large data amounts are communicated with each UE in this state, e.g. due to high load. On the other hand, the radio resources available in the CELL_FACH state are typically occupied briefly for transitory and sporadic transmissions when very small data amounts are communicated with each UE.
A UE with such “small” transmissions may still need to be constantly connected, e.g. when its uplink transmissions are quite frequent such as in so-called “always on” services. In that case, the UE would get better performance in the CELL_DCH state by not having to perform the random access process every time it has data to send which saves time and signaling, and also because this state supports a fast power control. However, precious radio resources may then be occupied over lengthy periods for the UE also when the UE has no data to send, which may contribute to congestion when the traffic is dense in the cell. Therefore, the UE is typically put in the CELL_FACH state instead to avoid congestion and reduced capacity in the cell.
In Release 8 of 3GPP, a concept called common Enhanced Dedicated Channel, E-DCH, was introduced which basically denotes a pool of shared radio resources that can be used temporarily by UEs in the CELL_FACH state when data is transmitted “sporadically” and in small packets from each UE. Such traffic of small data amounts may be identified as a “chatty” type of traffic or a machine type of traffic. The common E-DCH is used to move UEs with small data amounts to send from the CELL_DCH state to the CELL_FACH state. The common E-DCH resources may also be used by UEs in idle state.
It is deemed more efficient for the network to utilize such common E-DCH resources in the CELL_FACH state for “small” transmissions than to occupy CELL_DCH resources, at least in situations with dense traffic, thus offloading the congestion-sensitive CELL_DCH resources and utilizing better the potential capacity of the CELL_FACH resources. Further, the transmissions over common E-DCH resources allow for applying a higher order of modulation and/or a Hybrid Automatic Repeat Request, HARQ, process involving acknowledgment or non-acknowledgment of properly decoded data, which both can ensure efficient communication of data and increase uplink data throughput in the CELL_FACH state significantly. The usage of common E-DCH resources in a cell may be managed by a base station serving the cell which also regularly broadcasts configuration information of the common E-DCH resources to any UEs present in the cell.
A UE can obtain a common E-DCH resource in the CELL_FACH state by sending a randomly selected code and preamble signature on a channel called Physical Random Access Channel, PRACH. The base station then returns an acknowledgement of an allocated resource over a channel called Acquisition Indicator Channel, AICH. In this disclosure, the term “signature” will be used for short to represent any codes, preamble signatures or similar parameters that can be selected by UEs and used in a random access procedure in the manner described, e.g. on the PRACH.
Since there is only a limited number of such signatures available to UEs in a cell, there is a risk, particularly during high load, that two or more UEs in the cell happen to randomly select the same signature. In that case, it is not possible for the network to uniquely identify each UE by means of their randomly selected signatures and a collision may thus occur between two or more UEs since the network would allocate and acknowledge the same common E-DCH resource to these UEs.
To avoid a collision when multiple UEs access the network with the same signature, a collision resolution period has been introduced during which the UEs transmit Media Access Control, MAC, packets with a unique UE identifier called E-DCH Radio Network Temporary Identifier, E-RNTI, attached in the MAC header of each packet. Upon reception of these MAC packets, the network will assign a granted common E-DCH resource to the E-RNTI of one of the UEs and this assignment is signaled over a channel called E-DCH Absolute Grant Channel, E-AGCH. The granted resource may be signaled with an E-RNTI specific Cyclic Redundancy Check, CRC, attached which the UE is able to recognize. The other UEs that did not get the granted resource over E-AGCH will therefore terminate their data transmissions once the collision resolution period expires and try again later with a new random access attempt.
An E-DCH session for a “burst-like” data transmission from a UE in the CELL_FACH state would typically be short. The allocated common E-DCH resource will be released after the session ends to become available for other UEs in the CELL_FACH state. The next data transmission from the UE would require the UE to perform the above-described PRACH access with another randomly selected signature, which is completed after expiry of another collision resolution period.
As explained above, the limited number of signatures available in a cell may be a bottleneck for uplink transmissions in the CELL_FACH state since this scheme only allows a limited number of UE sessions to be active at the same time. In case of dense traffic in the cell, some UEs not getting a common E-DCH resource will have to wait for ongoing sessions to be released before starting a session. It is therefore desirable to release a session no later than necessary. On the other hand, it is more efficient to maintain a session for UEs that frequently send short bursts of data thus saving the time and signaling involved in the PRACH random access procedure.
Various schemes for dividing the available signatures into different partitions with respect to different UE capabilities and features, have been proposed where different subsets of the signatures are reserved for different partitions. In this way, it is possible to handle the UEs within each partition uniformly in terms of the random access process and assignment of radio resources. However, it is a problem that a blocking situation may occur when all available radio resources are occupied by a number of UEs in the cell such that no further UEs are able to obtain a radio resource for transmitting uplink data. Such a blocking situation may also occur within one or more of the above-mentioned partitions if there are currently too many UEs in the cell that match a certain partition in relation to the number of signatures reserved for that partition.